Induction heating for underfill removal and chip rework

BACKGROUND

0001The invention relates generally to semiconductor packaging and, in particular, to underfill materials and methods for removing an underfill material from beneath a chip in relation to removal of the chip from a substrate.

0002A die or chip includes integrated circuits formed by front-end-of-line processing using the semiconductor material of a wafer, a local interconnect level formed by middle-of-line processing, and stacked metallization levels of an interconnect structure formed by back-end-of-line processing. After the wafer is diced, each chip may be joined with a substrate using, for example, a controlled collapse chip connection or flip chip process. In a flip chip process, reflowed solder bumps establish mechanical and electrical connections between pads in the top metallization level of the interconnect structure and a complementary set of pads on the substrate. The solder bumps can be formed on the pads of the chip using any number of techniques, including electroplating, evaporation, printing, and direct placement. Reflow of the solder bumps establishes solder joints that physically and electrically connect the chip pads with the substrate pads.

0003Underfill may be applied to fill open space beneath the chip that remains between the solder joints. After curing, the underfill may function to protect the solder joints against various adverse environmental factors and redistribute mechanical stresses arising from shock. The underfill may also prevent the solder joints from shearing during thermal cycles. Coefficient of thermal expansion (CTE) mismatch between the chip and the substrate can cause mechanical stresses as temperature changes are experienced that can lead to solder joint shearing and reliability issues.

0004Improved underfill materials and methods for removing an underfill material from beneath a chip in relation to removal of the chip from a substrate are needed that improve on existing underfill materials and such removal methods.

SUMMARY

0005In an embodiment of the invention, a method is provided for removing an underfill material from beneath a chip. The underfill material is heated by induction heating. While the underfill material is heated, the underfill material is removed from beneath the chip.

0006In an embodiment of the invention, a method is provided for reworking a chip. An underfill material between the chip and a substrate is heated by induction heating. Solder bumps coupling a plurality of first pads on the chip with a plurality of second pads on the substrate are also heated. While the underfill material and the solder bumps are heated, the chip is removed from the substrate.

0007In an embodiment of the invention, an underfill material includes a plurality of particles dispersed in a bulk matrix. A material constituting the particles is configured to generate heat energy by induction heating when exposed to a time-varying magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

0008The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

0009<figref idref="DRAWINGS">FIG. 1</figref> is a side view of a chip mounted to a substrate by an array of solder balls and with underfill material in a space between the chip and the substrate.

0010<figref idref="DRAWINGS">FIG. 2</figref> is a diagrammatic view of a system in accordance with an embodiment of the invention.

0011<figref idref="DRAWINGS">FIG. 3</figref> is a side view similar to <figref idref="DRAWINGS">FIG. 1</figref> in which the underfill material is in the process of being removed from the space between the chip and the substrate with the assistance of induction heating through the operation of the system of <figref idref="DRAWINGS">FIG. 2</figref>.

DETAILED DESCRIPTION

0012With reference to <figref idref="DRAWINGS">FIG. 1</figref>, an assembly <b>10</b> includes a substrate <b>12</b> and a chip <b>14</b> that is mounted to a surface <b>16</b> of the substrate <b>12</b>. The chip <b>14</b> includes a plurality of pads <b>18</b> formed in a topmost metallization level of the interconnect structure and solder bumps <b>20</b> that are formed on, or placed onto, the pads <b>18</b>. The solder bumps <b>20</b> are configured to be reflowed to attach the chip <b>14</b> to the substrate <b>12</b>. In particular, each solder bump <b>20</b> mechanically attaches one of the chip pads <b>18</b> with one of a plurality of pads <b>22</b> on the substrate <b>12</b> in a rigid connection. In additional to the mechanical attachment, the solder bumps <b>20</b> and pads <b>18</b>, <b>22</b> provide electrical pathways for transferring signals between the integrated circuit(s) of the chip <b>14</b> and an external device, and also provide electrical pathways for powering and grounding the integrated circuit(s) of the chip <b>14</b>.

0013The solder bumps <b>20</b> may be separately formed and transferred to the chip pads <b>18</b> by a controlled collapse chip connection (C4) technology. Alternatively, each solder bump may be formed on its chip pad <b>18</b> by electroplating using an appropriate plating solution, an anode, a cathode, and a direct current supplied to the anode/cathode while in the plating solution. The solder bumps <b>20</b> may be comprised of solder having a lead-free (Pb-free) composition, a eutectic tin/lead (Sn/Pb) composition, a high lead (Pb) composition, etc. An assembly including the chip <b>14</b>, substrate <b>12</b>, and the solder bumps <b>20</b> may be heated in a reflow oven to melt the solder in the bumps <b>20</b>. Upon solidification, the solder bumps <b>20</b> have respective metallurgical bonds with the pads <b>18</b>, <b>22</b>.

0014The substrate <b>12</b> may be comprised of an organic material, such as a polymer or plastic, and the organic material may optionally be reinforced with, for example, glass fibers. Alternatively, the substrate <b>12</b> may be comprised of an inorganic material, such as a ceramic. The pads <b>18</b>, <b>22</b> may be comprised primarily of aluminum (Al) or copper (Cu), and may further include one or more layers of other materials, such as titanium tungsten (TiW), nickel (Ni), etc., comprising under-bump metallurgy (UBM).

0015Underfill material <b>24</b> may be introduced beneath the chip <b>14</b> into the space between the chip <b>14</b> and substrate <b>12</b> that is not occupied by the solder bumps <b>20</b>. The underfill process may entail applying the underfill material <b>24</b> as a fluid to the substrate <b>12</b> adjacent to the perimeter of the chip <b>14</b> and allowing capillary action to draw the underfill material <b>24</b> from the perimeter into the space between the chip <b>14</b> and the substrate <b>12</b>. When hardened by curing, the underfill material <b>24</b> forms a strongly-bonded, cohesive mass. Among other effects, the underfill material <b>24</b> protects the solder bumps <b>20</b> against various adverse environmental factors, redistributes mechanical stresses due to shock, and prevents the solder joints from shearing under strain experienced during thermal cycles due to CTE mismatch.

0016The underfill material <b>24</b> may comprise a bulk matrix <b>25</b> comprised of, for example, a thermoplastic material that is an electrical insulator and non-conductive. The bulk matrix <b>25</b> of the underfill material <b>24</b> may include one or more polymerizable monomers, polyurethane prepolymers, block copolymers, and radial copolymers, as well as substances like initiators, catalysts, cross-linking agents, stabilizers, etc. After the underfill material <b>24</b> is located beneath the chip <b>14</b>, the bulk matrix <b>25</b> of the underfill material <b>24</b> is cured and hardened. For example, thermoplastic materials may contain polymer molecules that can be chained or cross-linked by heat, ultraviolet light, or another type of electromagnetic energy to form a strongly bonded and cohesive mass.

0017The underfill material <b>24</b> may further include a plurality of particles <b>26</b> that are comprised of a material that is capable of coupling with a time-varying magnetic field to induce eddy currents and/or magnetic hysteresis losses in the particles <b>26</b> and to thereby cause induction heating by one or the other, or both, mechanisms. The particles <b>26</b> are dispersed or distributed in the bulk matrix <b>25</b> to form a composite material made from multiple constituent materials. The particles <b>26</b> are configured to respond by generating heat energy when exposed to a time-varying magnetic field by induction heating due to eddy currents and/or magnetic hysteresis losses, and with an associated temperature rise as the heat energy is generated. The bulk matrix <b>25</b> and any filler therein are not electrically conductive and, therefore, do not heat or otherwise generate heat energy in response to a time-varying magnetic field. The time variation of the magnetic field induces eddy currents eddy currents and/or magnetic hysteresis losses by electromagnetic induction in the material constituting the particles <b>26</b>. Due to the electrical resistance of an electrically-conducting material, eddy currents generate heat energy by Joule heating. The temperature of the particles <b>26</b> rises and heat energy flows outward from the particles <b>26</b> by thermal conduction into the surrounding cured bulk matrix <b>25</b> comprising the underfill material <b>24</b>. The heating rate of the particles <b>26</b> may be dependent, among other factors, on the frequency of the induced currents, the intensity of the induced currents, the specific heat of the constituent material, the magnetic permeability of the material, and the electrical resistance of the material to the flow of current.

0018In an embodiment, the particles <b>26</b> may be comprised of a ferriceramic material, such as a ferrimagnetic material like hematite (Fe₂O₃), magnetite (Fe₃O₄), or a ferrite (MFe₂O₄, where M is a divalent ion such as nickel, zinc, cadmium, manganese, or magnesium). Ferriceramic materials are electrical insulators characterized by a low electrical conductivity, yet heat when exposed to a time-varying magnetic field due to magnetic hysteresis losses. Alternatively, if the end use of the assembly <b>10</b> is tolerant to the addition of particles <b>26</b> that are electrically conductive to the underfill material <b>24</b> such that the dielectric properties of the bulk matrix <b>25</b> are reduced, the particles <b>26</b> may be comprised of a ferromagnetic material, such as iron, nickel, cobalt, or an alloy of these materials, that heat when exposed to a magnetic field due to eddy currents and/or magnetic hysteresis losses. Incidental to the ability to heat the bulk matrix <b>25</b> of the underfill material <b>24</b> with heat energy transferred from the particles <b>26</b> when immersed in a time-varying magnetic field, the particles <b>26</b> may function to improve the mechanical properties of the cured underfill material <b>24</b>. The particles <b>26</b> may be spherical or near spherical in shape, and may have a diameter or a distribution of diameters in the range of 1 micron to 10 microns. Alternatively, the particles <b>26</b> may be characterized by other shapes and sizes so long as the particles <b>26</b> have smaller dimensions than the solder bumps <b>20</b>.

0019Heat energy from the distributed heat sources represented by the heated particles <b>26</b> is transferred by thermal conduction to the surrounding bulk matrix <b>25</b> of the underfill material <b>24</b>. The transferred heat energy causes the temperature of the surrounding bulk matrix <b>25</b> to rise from room or ambient temperature to an elevated temperature that is greater than room or ambient temperature. In an embodiment, the heating of the underfill material <b>24</b> may be sufficient to produce an elevated temperature that induces a phase transition of the cured bulk matrix <b>25</b> of the underfill material <b>24</b> from a solid phase to a liquid phase or a semisolid state. As used herein, a semisolid state is characterized by a consistency and/or viscosity intermediate between the solid phase and the liquid phase of a material, and may also result as an outcome when the cured bulk matrix <b>25</b> is liquefied.

0020The induction heating of the assembly <b>10</b> is selective in that the underfill material <b>24</b> experiences a far greater temperature rise than ether the chip <b>14</b> or the substrate <b>12</b>. A benefit of the selective heating, among other benefits, is that the thermal aging of the substrate <b>12</b> may be reduced in comparison with a rework process that melts the underfill material <b>24</b> by heating the entire assembly <b>10</b>, for example, in a reflow oven or with a forced flow of heated air. While the underfill material <b>24</b> is in the liquid phase or the semisolid state, the underfill material <b>24</b> may be removed from the space between the chip <b>14</b> and substrate <b>12</b>, for example, by suction or by a forced flow of heated or room temperature air. The solder bumps <b>20</b> may be liquefied in a subsequent heating process in order to facilitate the release of the chip <b>14</b> from the substrate <b>12</b>. In this embodiment, the removal of the underfill material <b>24</b> and the liquefaction of the solder bumps <b>20</b> occur during the performance of different heating processes.

0021In another embodiment and in addition to increasing the temperature of the cured bulk matrix <b>25</b> of the underfill material <b>24</b>, the heating of the underfill material <b>24</b> may also be sufficient to cause the solder bumps <b>20</b> to experience a phase transition from a solid phase to a liquid phase or a semisolid state because of an increase in the temperature of solder bumps <b>20</b>. In order to be liquefied, the solder bumps <b>20</b> do not have to be comprised of a specially engineered solder material because the electromagnetic properties of the particles <b>26</b> dispersed in the bulk matrix <b>25</b> couple with the time-varying magnetic field to provide the heat source. The chip <b>14</b> may be removed from the substrate by, for example, a spider rework in which the substrate <b>12</b> and chip <b>14</b> are inverted with a weight attached to the chip <b>14</b>. When the solder bumps <b>20</b> and underfill material <b>24</b> are both liquefied, gravitational forces assist in causing the chip <b>14</b> to be released from the substrate <b>12</b>. In this embodiment, the removal of the underfill material <b>24</b> and the liquefaction of the solder bumps <b>20</b> occur during the performance of the same heating process.

0022The underfill material <b>24</b> may further include small particles of a filler material comprised of an electrical insulator, such as glass or silica, and dispersed in the bulk matrix <b>25</b> in addition to the particles <b>26</b>. Such optional filler particles may function to further improve the mechanical properties of the cured underfill material <b>24</b>, but do not heat when exposed to a time-varying magnetic field.

0023With reference to <figref idref="DRAWINGS">FIGS. 2, 3</figref> and in accordance with an embodiment of the invention, a system <b>30</b> is shown that is configured to use induction heating to cause the underfill material <b>24</b> to heat through the heating of the particles <b>26</b> and experience a temperature rise. The system <b>30</b> includes a controller <b>32</b> with at least one processor <b>34</b> including at least one hardware-based microprocessor and a memory <b>36</b> coupled to the at least one processor <b>34</b>. The memory <b>36</b> may represent the random access memory (RAM) devices comprising the main storage of controller <b>32</b>, as well as any supplemental levels of memory, e.g., cache memories, non-volatile or backup memories (e.g., programmable or flash memories), read-only memories, etc. In addition, memory <b>36</b> may be considered to include memory storage physically located elsewhere in the controller <b>32</b>, e.g., any cache memory in a microprocessor, as well as any storage capacity used as a virtual memory, e.g., as stored on a mass storage device or on another computer coupled to the controller <b>32</b>.

0024For interfacing with a user or operator, the controller <b>32</b> may include a user interface <b>38</b> incorporating one or more user input/output devices, e.g., a keyboard, a pointing device, a display, a printer, etc. Otherwise, input may be received via another computer or terminal over a network interface <b>40</b> coupled to a communication network. The controller <b>32</b> also may be in communication with one or more mass storage devices, which may be, for example, internal hard disk storage devices, external hard disk storage devices, external databases, storage area network devices, etc.

0025The controller <b>32</b> typically operates under the control of an operating system <b>42</b> and executes or otherwise relies upon various computer software applications, components, programs, objects, modules, engines, data structures, etc., including for example, a heater control module <b>44</b> and a removal control module <b>45</b>. The heater control module <b>44</b> may be configured to control the operation of a power supply <b>46</b> when its instructions are executed by the at least one processor <b>34</b> of the controller <b>32</b> in order to power an induction heater <b>48</b> that causes the underfill material <b>24</b> to be heated and experience a temperature rise above its initial temperature (e.g., room temperature). The controller <b>32</b> may include a power supply interface <b>50</b> that couples the controller <b>32</b> with the power supply <b>46</b>. The removal control module <b>45</b> may be configured to control the operation of a removal apparatus <b>60</b> when its instructions are executed by the at least one processor <b>34</b> of the controller <b>32</b> in order to operate a removal apparatus <b>60</b> that is configured to remove the underfill material <b>24</b>. The underfill material <b>24</b> may be removed either during heating of the underfill material <b>24</b> by the induction heater <b>48</b> or after heating of the underfill material <b>24</b> by the induction heater <b>48</b> while the underfill material <b>24</b> is in a condition (i.e., a liquid phase or a semisolid state) suitable for removal. The controller <b>32</b> may include a removal apparatus interface <b>56</b> that couples the controller <b>32</b> with the removal apparatus <b>60</b>.

0026Moreover, various applications, components, programs, objects, modules, etc. may also execute on one or more processors in another computer coupled to the controller <b>32</b> via the communication network, e.g., in a distributed or client-server computing environment, whereby the processing required to implement the functions of a computer program may be allocated to multiple computers over a network. The memory <b>36</b> may store one or more data structures including, for example, a database <b>52</b> configured with records <b>54</b> to store data relating to the process (e.g., control settings for the power supply <b>46</b>, control settings for the removal apparatus, etc.).

0027As best shown in <figref idref="DRAWINGS">FIG. 3</figref>, the induction heater <b>48</b> is located proximate to the assembly <b>10</b> at the time of use to remove the underfill material <b>24</b> from beneath the chip <b>14</b> in connection with reworking the chip <b>14</b>. In a representative embodiment, the induction heater <b>48</b> may comprise an induction coil <b>49</b> consisting of multiple turns of a helically-wound conductor. The time-varying magnetic field, as indicated diagrammatically by the field lines <b>58</b>, is formed in and around the turns of the induction coil <b>49</b>, when circulating a time-varying electrical current through the induction coil <b>49</b>, consistent with the Biot-Savart law. The strength of the magnetic field generated by the induction heater <b>48</b> varies with distance from the induction coil <b>49</b> and may be on the order of one (1) Tesla, although other field strengths may be applicable.

0028The induction coil <b>49</b> of the induction heater <b>48</b> is coupled by, for example, a high voltage cable with the power supply <b>46</b>, which in turn is in communication with the controller <b>32</b> through the power supply interface <b>50</b>. Time-varying electrical power is supplied from the power supply <b>46</b> to the induction coil <b>49</b> of the induction heater <b>48</b> in response to program code executed by the at least one processor <b>34</b>, user interaction with the user interface <b>38</b>, and/or other instructions or input received by the at least one processor <b>34</b>. The power supply <b>46</b> may supply high-frequency alternating current to the induction heater <b>48</b>. In one embodiment, the alternating current may be supplied to the induction heater <b>48</b> at a high frequency, such as at a radio frequency (e.g., 13.6 MHz). The frequency of the alternating current generating the magnetic field and the size of the particles <b>26</b> may be varied, among other factors, to modify the specific characteristics of the heating.

0029The assembly <b>10</b> can be positioned relative to the induction heater <b>48</b> such that the underfill material <b>24</b> between the substrate <b>12</b> and chip <b>14</b> is subjected to and influenced by the time-varying magnetic field <b>58</b> emanating from the induction coil <b>49</b>. The induction coil <b>49</b> of the induction heater <b>48</b> may be dimensioned to receive the chip <b>14</b> inside its inner diameter. In a representative embodiment, the induction coil <b>49</b> of the induction heater <b>48</b> may have an inner diameter on the order of two (2) inches to three (3) inches with four (4) to five (5) turns and may be cooled by a cooling medium flowing through a lumen of the induction coil <b>49</b>. Contact is not required between the chip <b>14</b> and the induction coil <b>49</b>.

0030The removal apparatus <b>60</b> of the system <b>30</b> is configured to remove the underfill material <b>24</b>, once converted to a liquid phase or semisolid state, from beneath the chip <b>14</b>. The removal apparatus <b>60</b> may be configured to direct pressured gas or air (e.g., air jets) at, for example, one or more side edges of the chip <b>14</b> and thereby generate a removal force to displace the liquefied underfill material toward opposite side edges of the chip <b>14</b>. The pressured gas or air may be maintained until the space beneath the chip <b>14</b> is effectively cleared of the underfill material <b>24</b>. Alternatively, the removal apparatus <b>60</b> may be configured to apply suction at one or more side edges of the chip <b>14</b> and thereby generate a removal force to displace the underfill material <b>24</b> toward those side edges. The applied suction may be maintained until the space beneath the chip <b>14</b> is effectively cleared of the underfill material <b>24</b>.

0031In use and with reference to <figref idref="DRAWINGS">FIGS. 1-3</figref>, the assembly <b>10</b> and the induction heater <b>48</b> are arranged such that the cured underfill material <b>24</b> can be exposed to the time-varying magnetic field generated by current flowing through the induction heater <b>48</b>. In one embodiment, the induction coil <b>49</b> is placed around the chip <b>14</b> to provide a surrounding arrangement. The controller <b>32</b> then energizes (or is caused to energize) the induction heater <b>48</b> by causing the power supply <b>46</b> to supply a time-varying electrical current to the induction coil <b>49</b> of the induction heater <b>48</b>. The time-varying electrical current in the induction coil <b>49</b> produces a time-varying magnetic field in the space in and about the induction coil <b>49</b> of the induction heater <b>48</b>. At any given point in space near the chip <b>14</b>, the magnetic field may be specified by a direction and a magnitude (or strength).

0032The time variation of the magnetic field induces eddy currents and/or magnetic hysteresis losses by electromagnetic induction in the material comprising the particles <b>26</b>. Due to the electrical resistance of the material, the electrical currents generate heat by Joule heating and the temperature of the particles <b>26</b> rises. Heat energy is transferred outward from the heated particles <b>26</b> by thermal conduction into the surrounding cured bulk matrix <b>25</b> comprising the underfill material <b>24</b>.

0033The heating time and parameters for the high-frequency alternating current supplied to the induction heater <b>48</b> are selected to transform the cured bulk matrix <b>25</b> from the solid phase to a liquid phase or semisolid state. Specifically, the cured bulk matrix <b>25</b> may be heated by the transferred heat energy to a temperature that is greater than or equal to a liquefaction point. The specific liquefaction point is contingent, among other factors, upon the composition of the cured bulk matrix <b>25</b>. The heating time required to liquefy the cured bulk matrix <b>25</b> may be on the order of tens to hundreds of microseconds. During the operation of the induction heater <b>48</b>, the heating of the chip <b>14</b> and the substrate <b>12</b> may be negligible.

0034While the bulk matrix <b>25</b> of the underfill material <b>24</b> is liquefied by a phase transformation from its cured solid phase to a liquid phase or a semifluid state, the removal apparatus <b>60</b> is operated to remove the underfill material <b>24</b> from beneath the chip <b>14</b>. After the underfill material <b>24</b> is removed, still liquefied in advance of removal, or in the process of being removed, the controller <b>32</b> may operate the power supply <b>46</b> to discontinue the time-varying current in the induction coil <b>49</b> of the induction heater <b>48</b>, and thereby discontinue the application of the time-varying magnetic field. After the underfill material <b>24</b> is removed, the chip <b>14</b> may be removed or separated from the substrate <b>12</b> in a subsequent step that involves liquefying the solder bumps <b>20</b>, for example, in a reflow oven or with heated air jets.

0035In an alternative embodiment, the solder bumps <b>20</b> on the chip <b>14</b> may be heated by the heat energy generated from the particles <b>26</b> in the underfill material <b>24</b> to a temperature that is greater than the melting point of the constituent solder material, which liquefies the solder bumps <b>20</b>. Specifically, heat energy is transferred outward from the cured bulk matrix <b>25</b> of the underfill material <b>24</b> by thermal conduction to the solder bumps <b>20</b>. The degree of heating may be controlled through, among other factors, selection of the properties on the particles <b>26</b> and the strength of the magnetic field. While the underfill material <b>24</b> and the solder bumps <b>20</b> are both in the liquefied state, the chip <b>14</b> is separated from the substrate <b>12</b>.

0036The cured bulk matrix <b>25</b> of the underfill material <b>24</b> and the solder bumps <b>20</b> each possess a given melting temperature. Though a selection of the constituent materials, the bulk matrix <b>25</b> of the underfill material <b>24</b> may be caused to liquefy and transform to the liquid phase or semisolid state prior to the liquefaction of the solder bumps <b>20</b> (i.e., the melting temperature of the bulk matrix <b>25</b> is less than the melting temperature of the solder bumps <b>20</b>). Alternatively, the bulk matrix <b>25</b> of the underfill material <b>24</b> may be caused to liquefy after the liquefaction of the solder bumps <b>20</b> (i.e., the melting temperature of the bulk matrix <b>25</b> is greater than the melting temperature of the solder bumps <b>20</b>).

0037Removing the underfill material <b>24</b> using induction heating may reduce the impact of the heating on the material constituting the substrate <b>12</b>. For example, the use of induction heating may eliminate wear-out mechanisms associated with the exposure of certain type of substrates <b>12</b>, such as plastic laminates, to an excessive number of heat cycles. These wear-out mechanisms may limit the number of permitted reworks of the solder bumps <b>20</b>.

0038The methods as described above are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. The chip may be integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

0039A feature may be “connected” or “coupled” to or with another element may be directly connected or coupled to the other element or, instead, one or more intervening elements may be present. A feature may be “directly connected” or “directly coupled” to another element if intervening elements are absent. A feature may be “indirectly connected” or “indirectly coupled” to another element if at least one intervening element is present.

0040The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.